Many years ago, shortly after the "deep dimensions" tanks came out (and I wanted one), I asked about the tanks and sure enough just like Robert says... good luck. The way it was is one particular distributor (who shall remain nameless) that had a literal monopoly over this entire area (not just the bay area) refused to sell them, because they didn't want any competition for their own large tanks (sister company or parent company or something), this was the story relayed to me while I stood in the LFS while the owner called Central Pet (why a CP rep would give this info out I don't know). Anyways, I'm not sure how much, if any, of their policy changed since then, but I wouldn't be surprised if a similar"embargo" of Marineland tanks is still in place. Oddly enough they have no problem stocking Marineland's other products.
So options for a Marineland tank
1) If you're planning a road trip out of state (way out of state) and have the ability to store it in the car/truck, call a few LFSs in the direction you're going.
2) Dr Foster's & Smith carry certain ML tanks, they seem pricey, but I have no gauge to how much they should cost, and very often the price you see is the tank/stand/setup price, you often have to dig a little deeper for the tank-only price, do note that shipping might be pricey.
3) Ditch the idea of a Marineland tank, find a stock tank in a similar dimension that you want that you can get locally.